I'm looking to take a signal (from a car ECU) to a MOSFET (it is PWM), then if the MOSFET fails (i.e. due to inadequate heatsinking, or else), that it fails in a closed state so that the output (Radiator Fans) stay on.

Can I sink the heat off the MOSFET to the car frame, given that it is also ground, or will that affect operation?

Heat is produced when they dissipate power.that is when the product of voltage dropped across them times current through them is at a maximum. Not turning them on fully is only one situation, not turning them on fast enough is another, especially in thisng like PWM.

How much power is being disapaited ? I was thinking maybe a sense resistor to determine if the fet is still behaving, by enabling, disabling you can measure the voltage drop via a sense resistor and if nothing changes signal a warning.

How much power is being disapaited ? I was thinking maybe a sense resistor to determine if the fet is still behaving, by enabling, disabling you can measure the voltage drop via a sense resistor and if nothing changes signal a warning.

If it's for a car radiator then maybe the existing temperature sensor is enough...

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It is but you will never have 8A flowing through the FET with 14V across it.When there is 14V across it there is no current down it.When there is 8A flowing through it the voltage across it is determined by the "on resistance" of the FET. This will be in the data sheet. Suppose it is 0.05R then 8A will give 0.4V across the FET. Therefore the FET will be dissipating 0.4 * 8 = 3.2W.

80.4 deg C is less than the datasheet Operating Junction Max Temp of 170 deg C.It's Junction to Ambient is 62 deg C/W.

No heatsink needed - or have I got it completely backwards?

If it was dissipating 112W without a heatsink then the temperature to ambient wouldbe 62 * 112 = 6.7kW !! But of course its not dissipating 112W.

You have to add the thermal resistances together, and if there is no heatsink you haveto add the 62 C/W (otherwise you add the heatsink's thermal resistance to ambient).The 0.45 C/W is really only relevant if you bolt the device onto a water-cooled mass ofcopper.

During switching the MOSFET might briefly be carrying about 4A at 7V (assuminga resistive load) when the channel resistance matches the load resistance, and this isthe maximum dissipation during switching (28W or 25% of the full load). However thismight only be for 1us, so its only an issue when the switching is at a high frequencyso that the proportion of time spent switching is non-negligible.

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